A stent is a generally longitudinal cylindrical device formed of biocompatible material, such as metal or plastic, which is used in the treatment of stenosis, strictures, or aneurysms in body blood vessels and other tubular body structures, such as the esophagus, bile ducts, urinary tract, intestines or the tracheo-bronchial tree.
A stent is held in a reduced diameter unexpanded configuration within a low profile catheter until delivered to the desired location in the tubular structure, most commonly a blood vessel, whereupon the stent radially expands to an expanded diameter configuration in the larger diameter vessel to hold the vessel open. Radial expansion may be accomplished by manually inflating a balloon which is attached to a catheter in a balloon expanding stent, or the stent may be of the self-expanding type that will radially expand spontaneously once released from the end portion of the delivery catheter.
Generally, self-expanding stents are made from materials that exhibit superelastic properties above a particular transitional temperature, that is, the material rapidly expands once the material is heated above the transitional temperature. For example, most of the currently available self-expanding stents are made from Nitinol which has a transitional temperature from the martensite to austenite state around 20° C. Thus, once a Nitinol stent reaches a temperature of 20° C. the stent will begin to rapidly expand in the radial direction. During the delivery of such stents within the vessel to be treated, the stent reaches the transitional temperature almost as soon as the stent is introduced into the body, i.e. well before the stent is actually released into the vessel and comes into contact with the bloodstream. Accordingly, the stent is maintained in a compressed state within the delivery catheter until the stent is positioned within the desired delivery location. When the stent is released into the vessel, since it has already reached its transitional temperature, and is in a compressed state, it will expand almost instantaneously to its maximum radial diameter.
The present invention relates to self-expanding stents, examples of self-expanding stent designs are shown in U.S. Pat. No. 5,064,435 to Porter; U.S. Pat. No. 5,354,308 to Simon; U.S. Pat. No. 5,569,295 to Lam; U.S. Pat. No. 5,716,393 to Lindenberg; U.S. Pat. No. 5,746,765 to Kleshinski; U.S. Pat. No. 5,807,404 to Richter et al; U.S. Pat. No. 5,836,966 to St. Germain; U.S. Pat. No. 5,938,697 to Killion; U.S. Pat. No. 6,146,403 to St. Germain; U.S. Pat. No. 6,159,238 to Killion; U.S. Pat. No. 6,187,034 to Frantzen; U.S. Pat. No. 6,231,598 to Berry et al.; U.S. Pat. No. 6,106,548 to Roubin et al.; U.S. Pat. No. 6,066,168 to Lau et al.; U.S. Pat. No. 6,325,825 to Kula et al.; U.S. Pat. No. 6,348,065 to Brown et al.; U.S. Pat. No. 6,355,057 to DeMarais et al and U.S. Pat. No. 6,355,059 to Richter et al.
Typically, a self-expanding stent is introduced into a vessel to be treated via a catheter delivery system which contains the compressed stent. The compressed stent is contained within a closed space or chamber at the distal end of the catheter delivery system, the chamber being defined between an inner core member and retractable sheath or sleeve. The retractable sheath functions to maintain the stent in the compressed state until the stent is positioned in its desired location. The catheter delivery system is provided with a lumen through which a guidewire is passed to enable the positioning of the stent containing chamber to the treatment site within the vessel. Once the stent is positioned in the desired treatment site within the vessel, the retractable sheath is retracted in a proximal direction to expose:the stent, which expands almost instantaneously to its final diameter as the sheath is being withdrawn. The stent is deployed into the vessel when the sleeve is completely retracted. Delivery systems of this type are described in U.S. Pat. Nos. 6,391,050; 6,375,676; and 5,77,669 which are each discussed in greater detail below.
U.S. Pat. No. 6,391,050 discloses a self-expanding stent delivery system which includes a catheter having an outer tube, the outer tube containing a channel for a guidewire lumen containing a guidewire arranged therein and a pullwire lumen containing a pullwire arranged therein. A distal end of the outer tube is affixed to a dual lumen tube which includes an inner lumen and a pullwire lumen, the guidewire lumen being arranged within the inner lumen. The pullwire lumen is provided with an axial slit from which the pullwire exits, the pullwire being coupled to a retractable sheath of a stent. When the pullwire is retracted the sheath is retracted thereby first exposing a distal end of the stent, then a middle area of the stent and finally the proximal end of the stent, thereby enabling the stent to expand.
U.S. Pat. No. 6,375,676 (“the '676 patent) describes a self-expanding stent and associated stent delivery system. The stent deliver system described in the '676 patent includes a delivery catheter including an inner tubular member which extends within an outer tubular member in a coaxial arrangement. The outer tubular member has a proximal end that is attached to a pull back handle that is designed to move axially. The distal end of the outer tubular member is provided with a flexible restraining sheath which is coupled to the outer tubular member, the sheath being adapted to maintain a stent in a collapsed state until the sheath is retracted. The sheath may be retracted, i.e. moved in a proximal direction, to enable the stent to expand by manually grasping the pull back handle. When the sheath is retracted a distal end region of the stent is exposed first, as the sheath continues to be retracted an intermediate region of the stent is exposed and finally a proximal region of the stent is exposed.
U.S. Pat. No. 5,772,669 (“the '669 patent”) discloses a stent delivery system for self expanding stents including a catheter having a stent containing portion near a distal end ofthe system in which a stent is arranged. The stent delivery system further comprises a proximal outer sheath, a retractable distal sheath which surrounds the stent and a pull back wire that is coupled to the retractable distal sheath. When the pull back wire is pulled proximally the distal sheath is retracted, a distal region of the stent is exposed first, then a middle region of the stent and finally the proximal region of the stent, thereby enabling the stent to fully expand.
A problem in stent delivery systems of the type described above results from the common feature that the stent is gradually exposed in relatively slow unidirectional manner from the distal end region of the stent towards proximal end region of the stent as the outer sheath is retracted. This problem with prior art stent delivery systems manifests itself in a number of clinical problems during deployment of the stent. These problems are discussed in greater detail below.
First, the exposed and expanded distal end of the stent may contact the wall of the vessel and serving as an “anchoring edge” for the remaining stent. In its fully expanded state this edge can no longer be moved, since any movement will be associated with irritation and trauma to the vessel wall. The trauma is caused by the metallic stent edges moving against the delicate inner surface of the vessel wall. Therefore, when using most of the currently available stent designs, once the distal end of the stent is fully expanded the stent is essentially locked into a position and further repositioning of the stent becomes impossible.
Another problem encountered in clinical practice, is related to a “spring effect” in which the retraction of the sheath over the self-expanding stent can cause the stent to behave like a spring. That is, as the sheath is removed, the stored energy in the compressed stent is suddenly released essentially causing the stent to move in a distal direction in an unpredictable way. This effect is exaggerated in stents having a short overall length. In such stents, when the distal end is exposed and becomes flared, but has not yet made contact with the vessel wall, and the proximal end of the stent is covered by only a few millimeters of the outer sheath, and then the sleeve is completely retracted, the stent tends to “jump” in a distal direction and will thus likely end up in the wrong final location.
Another difficulty which may be encountered during the release of self-expanding stents when using conventional delivery systems is related to a “push-pull” phenomenon. The “push-pull” phenomenon is encountered most dramatically in long catheter delivery systems, which are placed in tortuous vessels. The “push-pull” phenomenon presents itself as forward motion of the inner core in relation to the retractable sheath at the distal end of the system, as the sheath is being retracted to expose the stent. In practical terms, it may result in a forward motion of the stent during its deployment thereby resulting in the inaccurate positioning of the stent.
The above discussed problems in prior art stent delivery systems make the accurate positioning of self-expanding stents within the desired location difficult.
It is therefore an object of the present invention to provide a new and improved stent delivery system that overcomes the shortcomings of the prior art stent delivery systems.
It is another object of the present invention to provide a new and improved stent delivery system that minimizes stent movement during deployment of the stent thereby enabling the accurate positioning of the stent within the vessel to be treated.
It is yet another object of the present invention to provide a new and improved stent delivery system that prevents/minimizes the spring effect, and jumping of the stent caused by the spring effect, when the stent is deployed.
It is yet another object of the present invention to provide a new and improved stent delivery system that minimizes the “push-pull” phenomenon.